Magnetic materials play an important role in modern telecommunication, computing, and information storage technology. Ultra-small magnetic particles are needed for the manufacture of ferro-fluids, i.e. colloidal suspensions of magnetic particles. Particles that are paramagnetic have applications in magnetic refrigeration, color imaging, and biological detection and separation processes. In particular, paramagnetic microparticles that are functionalized with specific binding moieties are increasingly used for cell separation due to the high efficiency, high cell viability, and low cost of this process and have been proposed for use in various schemes for pathogen detection.
The magnetic particles presently used in ferro-fluid and bio-magnetic applications suffer from a number of deficiencies that limit their utility. These magnetic carriers make use of magnetic iron oxides usually embedded in a polymer matrix. They are not nano-size (typically micron size or larger), are characterized by low magnetic susceptibility that makes them unsuitable for many applications, and are susceptible to loss of their magnetic properties due to chemical transformation of magnetic iron oxides to diamagnetic Fe2O3.
Metal nanoparticles with their smaller size and higher magnetic susceptibility would be expected to possess a number of advantages over the current commercial particles, especially in bio-magnetic applications. They have a lower surface area which would reduce non-specific protein binding and since many immunological responses rely on surface antigen recognition they would be expected to produce reduced immunological response. Their higher magnetic susceptibility means that smaller magnetic fields would be required to manipulate them.
In many ways the ideal particle for bio-magnetic applications would be a nano-size particle that combines the high magnetic susceptibility of metallic iron with the well developed chemistry for attaching bio-active moieties and the resistance to oxidation of metallic gold. O'Connor and coworkers have reported the microemulsion synthesis of nanoparticles composed of a central core of Fe atoms surrounded by a 2–3 nm thick shell of Au atoms that purport to have these very characteristics (the so-called “Fe@Au particles”, J. Solid State Chem. 159, 26–31 (2001); U.S. Pat. Appl. 20020068187, published Jun. 6, 2002). These nanoparticles are synthesized in a two step process in a reverse micelle. If the gold shell perfectly encapsulates the iron core of a Fe@Au particle, the Fe atoms will be protected from oxidation, however, Ravel et al. (J. Appl. Phys. 91, 8195–8197 (2002)) have found that on average most of the Fe atoms in Fe@Au particles are oxidized, most probably due to the imperfect encapsulation of the Fe cores. This results in a lower magnetic susceptibility and non-uniform magnetic properties among a population of Fe@Au particles nominally of a given size and Fe composition. A second drawback of Fe@Au particles for bio-magnetic applications is the difficulty of removing all traces of the surfactant species used in their synthesis and replacing them with selected bio-active moieties without causing aggregation of the nanoparticles. Thus, while the Fe@Au particle represents a substantial advance over current magnetic carriers, there is still a need to improve on the Fe@Au particle for bio-magnetic applications.
There remains a need for improved, readily synthesized nanoscale magnetic particles, especially particles whose magnetic moment can be precisely controlled, and particles that can be readily modified by attachment of selected organic molecules to their surfaces.